1. Field of the Invention
The present invention relates to a flexible printed circuit board (FPC), and more particularly, to an interlayer connection structure of a multiplayer wiring board, which permits manufacturing of FPC at low cost with the equivalent performance to FPC with a conductive pattern formed by etching and which permits forming of land at a desired position to connect each board of a multiplayer wiring board made by laminating a plurality of flexible printed circuit boards, and a method of manufacturing a flexible printed circuit board, and a method of forming a land thereof.
2. Description of the Related Art
Current modular design trend in electrical equipment for motor vehicles spreading in Europe and America has advanced from an assembly type module connecting already-existing units toward an integrated module combining the functions of each unit and accessory. The module structure will finally develop into amalgam in which units, and accessories and a wire harness connecting them are integrated in a single-piece structure. For realizing such a module structure, a flexible printed circuit board (FPC) is highly contributive. FPC permits mounting of units, accessories, switches and other circuit components, and is lightweight and enables high-density wiring. Therefore, FPC is considered the shortest way to the interlayer connection in an amalgam type module.
A flexible printed circuit board is usually configured as shown in FIGS. 35A to 35C attached hereto. That is, FPC 100 is constructed by that a conductive pattern 102 made of copper foil is laminated by adhesive 104 on a base material 101 composed of polyester film (PET), polyamide film (PI) or the like, and a cover lay 103 made of synthetic resign is coated on the base layer through adhesive 105 to protect and insulate the conductors.
FIGS. 36 and 37 are flow charts showing a processes of manufacturing the FPC 100. In a so-called pre-process, as shown in FIG. 36, a step of smoothing surface (S11) including cleaning the surface of a copper foil is performed first, and a dry film laminating step (S12) follows to laminate a copper foil and a dry film as a base material. This laminating step is a process of forming a so-called copper-clad laminate 200, in which a copper foil 106 is placed on a base material 101 with adhesive 104 coated thereon, and heated and pressed by work rolls 301a and 301b of a heat roller rolling on both sides, thereby the copper foil is heated, pressed, laminated, dried and cured, as shown in FIG. 38. To form a wiring pattern on the copper-clad laminate 200 produced through the above process, first the surface of the copper foil 106 is coated with a resist, and rendered to exposing (S13), developing (S14), modifying (S15) and etching (S16). A predetermined conductive pattern 102 is formed through these steps, and submitted to a final step of intermediate inspection (S17).
In a so-called post-process, the surface of the conductive pattern 102 is polished (S21), and a cover lay film is laminated (S22) to protect the surface. This cover lay film laminating step is a process of forming FPC 100, in which a cover lay film 103 with adhesive 105 coated underside thereof is placed on the copper-clad laminate 201 having a conductive pattern 102 formed thereon, and heated, pressed and adhered by work rolls 301a and 301b of a heat roller rolling on both sides, as shown in FIG. 39. Thereafter, the copper-clad laminate is cured at a predetermined temperature (S23), the portion of the conductive patter 102 not covered by a cover lay is plated (S24), for example, and rendered to punching (S25), trimming (S26), and blanking (S27), and submitted to a final step of products. The FPC 100 is completed in this way.
However, the above described prior art method of manufacturing FPC needs etching to form a copper-clad laminate 200. This increases the number of manufacturing steps, decreases the yields of materials, and requires high cost of waste liquid disposing facilities. Thus, FPC itself becomes very expensive. Moreover, in cases when FPC is used for wiring of modules, a copper foil with a certain thickness must be used to meet a so-called medium-current circuit. Etching of a thick copper foil decreases reliability, and increases material and processing costs. Therefore, it will result in increased cost to merely use FPC instead of the wiring used in current modules. This has been a problem in using FPC for wiring of modules. Furthermore, when FPC is used as a harness or joint box in motor vehicles, a large current value in a circuit requires a copper foil with the thickness larger than a predetermined value. Generally, thicker the copper foil, the material cost will become high, and the total production cost will rise. This is another problem in using FPC. In addition, If FPC is produced by etching, a thicker copper foil requires longer etching time, and the processing cost will rise. Especially, if FPC for use in a large-current circuit is produced by a method including etching as described above, increase in cost will become serious.
Further, when FPC 100 is applied to a joint box, for example, usually a plurality of FPCS are laminated to form a multiplayer wiring board and the board is contained in a joint box, and interlayer connection to connect each layer will be necessary. However, in FPC, a base material 101 and a cover lay 103 are adhered to a conductive pattern 102 from the bottom and top thereof, respectively, and it will be necessary to make a land 106 for interlayer connection outside a circuit 107, as shown in FIG. 40A, and connect only the conductive patterns 102 by eliminating a base material 101 and cover lay 103, as shown in FIG. 40B. Thus, the dimension of FPC 100 circuit will be inevitably long. This is still another problem in the prior art method of manufacturing FPC.